Visbreaking is a long established petroleum refining process for the mild conversion by thermal cracking of heavy oil feeds such as vacuum resid which was originally developed for reducing the pour points and viscosities of residual stocks used in the manufacture of fuel oil so as to reduce the amount of cutter stock required to dilute the resid to meet fuel product specifications. Visbreaking may also be used to increase feeds stocks for FCC use and gasoline yields.
The visbreaking process operates by thermally cracking viscous, high molecular weight hydrocarbons into lighter, less viscous products although conversion does not achieve high levels. The process is essentially, a mild thermal cracking without substantial coke production, although visbreakers do need to be periodically decoked to remove the coke deposits which progressively accumulate over long periods in normal operation. It is, however, the limitation on coke formation which marks out visbreaking as a mild, usually liquid-phase thermal cracking operation, distinct from higher severity thermal cracking processes such as delayed coking which result in a much larger proportion of the feed being converted to coke. For most feedstocks, the conversion to light products (gasoline and lighter) is normally limited to about 10 percent; up to some 20 percent will normally go to total light distillate (e.g. 370° C.−) fractions with the remainder in the gas oil range, e.g. the non-residual (distillable) 370° C.+ fraction. Because the resid feeds commonly used have greater coking tendencies than gas oils, lower temperatures are normally used in visbreaking than in thermal cracking; typically ranging from about 455 to 525° C. (850 to 975° F.) under pressures up to 7000 kPag (about 1000 psig), typically 350 to 2100 kPag (about 50 to 300 psig) but commonly in the range 350 to 700 kPag (50 to 100 psig).
Visbreaker units are generally of two types, soaker or coil. The soaker type unit uses a heater ahead of a heat soak drum which is a tank-like vessel fitted with internals to reduce back mixing and improve plug flow, in which the heated feed is held for a time sufficient to enable the desired degree of thermally-induced cracking to proceed. The coil visbreaker normally has a two-zone fired heater with the reaction zone formed by furnace coils through which the feed passes in plug flow. The soaker visbreaker operates at lower temperatures with longer residence times than the coil type visbreaker and, as a result, tends to have a lower energy requirement. The coil visbreaker, however, enables better control of the reaction conditions with varying feeds and can, moreover, be more easily decoked by steam-air decoking. In both cases, however, the cracking reactions are terminated by quenching and no resort is made to recycle. Typical furnace outlet temperatures for a coil unit would be 475 to 500° C. (about 885-930° F.) with a residence time in the coil of one to three minutes whereas a soaker unit would use a lower furnace outlet temperature of 425-450° C. (about 800-840° F.).
Visbreaker feeds generally comprise high molecular weight paraffins, aromatics, asphaltenes, as well as aromatics and asphaltenes with paraffinic side chains. These feedstreams are usually highly viscous with viscosities generally from about 20 to about 1500 centistokes at 100° C. (212° F.). Typical heavy hydrocarbon feedstreams to the visbreaking process are those that have an initial boiling point above 315° C. (about 600° F.), more preferably above about 425° C. (about 800° F.). Common visbreaker feeds may be comprised of crude atmospheric tower bottoms, crude vacuum tower gas oils and/or crude vacuum tower bottoms.
Preferably, a significant amount of products can be converted into the naphtha boiling range products with a typical boiling range of about 25° to 250° C. (about 80° F. to about 450° F.), and distillate to gas oil range products typically boiling in the range of 1750 to 425° C. (about 350° F. to about 800° F.). Due to the limited severity of visbreaker operation, the visbroked liquid product will include a large proportion in the heavy gas oil range (about 290° to about 425° C./about 550° to about 800° F.) as well as visbreaker bottoms which generally have boiling points above 400° C. (about 750° F.), more typically above about 425° C. (about 800° F.). Excessive severity with its greater conversion to lighter products can, however, lead to several problems in the visbreaking process. The first is an overabundance of light gases in the product. These light gas products are generally of low economic value and are therefore undesired. Secondly, high severities can result in excessive formation of coke in the visbreaking unit and fouling deposits, in the form of toluene insolubles, in the liquid product. Although facilities and operating conditions may minimize as well as remove some of the coke formation in the unit. As a result, visbreaker units must be taken out of service at periodic intervals in order to remove the coke that forms in the unit. For these reasons, it is desirable to run the visbreaker unit within controlled ranges of severity.
High visbreaker severity can result in the formation of highly aromatic product streams. These streams may be of limited value for use in commercial fuel products due to restrictions on aromatic fuel contents and may also cause the fuel products to be excessively unstable: the products may polymerize and develop waxes bringing the desired products out of required fuel specifications as well as causing plugging problems in associated equipment.
Heavy gas oil range products from the visbreaker containing significant amounts of aromatic hydrocarbons may be catalytically cracked further into lighter fuels such as naphthas or gasolines, but excessive coke formation on the cracking catalysts (e.g., a fluid catalytic cracking or hydrocracking catalyst) is a common consequence, resulting in decreased catalytic activity, as well as increased unwanted processing unit emissions (such as CO and CO2 from FCC regenerators) or increased catalyst regeneration in hydroprocessing units.
Similarly, the visbreaker bottoms product stream may possesses similar undesirable properties arising from a high aromatic content. A significant amount of the aromatic content of the visbreaker bottoms product stream is in the form of asphaltenes with their characteristic high Conradson Carbon Residue (CCR) number. The high asphaltene content and high CCR number of the visbreaker bottoms stream render it prohibitive to process this stream further catalytically and for this reason, the visbreaker bottoms stream is usually thermally cracked in a resid conversion unit such as a coker unit or diluted as required for sale as fuel oils. Both the visbreaker gas oil product and the visbreaker bottoms product, however, contain significant amounts of valuable high molecular weight saturated hydrocarbons with relatively low CCR content although they cannot be removed from the undesired highly aromatic, high CCR hydrocarbons by conventional fractionation techniques.
There is therefore a need to improve visbreaker operation to enable the operational severity to be increased without increasing, at the same time, the formation of coke and other fouling products